This is a U.S. national stage patent application based on International Patent Application PCT/EP94/01043, filed on Apr. 3, 1994 and claiming priority dates of Apr. 6, 1993 (filing date of German patent DE 43-11978-C1) and Jan. 25, 1994 (filing date of German patent DE 44-01974-A1).
1. Field of the Invention
The present invention discloses method and apparatus for a dent stiffening process for sheet material or foil in which the wall is curved at regular distances.
2. Background Art
For economic and material reduction purposes thin-walled equipment or components are required in numerous technical applications which nevertheless have to have good strength, or shape stability. As these equipment are often component parts utilized in energy and environmental related applications, the walls should ideally have favorable inflow and heat-transmission properties. For weight and economic reasons, thin-walled and dimensionally stable constructions are also required in the packaging, design, interior fittings and building trades. In addition to structural rigidity, foil or thin-walled equipment should also have good optical features.
There are numerous known deformation processes which produce thin-walled materials with an increased structural rigidity. A well-known example is the beaded seam in cans or drums. Beads have the disadvantage that they only achieve a one-dimensional structural rigidity. When a multi-dimensional structural rigidity is required, the process requires sophisticated multi-dimensional matrix molds.
One drawback of the present profiling technology is that normally the wall deformation is achieved by mechanical means, where rolling or impressing is applied, or by hydraulic means, where pressure is put on a matrix. This alters the wall thickness while the original smooth wall surface quality is degraded. Only when the mold has a smooth surface, which normally means that its manufacture was very sophisticated, can a smooth surface of the deformed wall be achieved.
It is known from published German patent application # DE-OS 25 57 215 that there is a hydraulic molding technology in which thin-walled tubes or cylindrical containers obtain a uniformly staggered dent structure. According to that patent, the interior cylindrical walls are supported by thrust rings or a helix and then excess pressure is externally applied to achieve the dent deformation. In this type of hydraulic profiling technology, which is quasi-exempt from mechanical contact, a high quality surface finish is achieved. The uniformly staggered dent structure of the tube walls results in an increase in the rigidity compared to the non-deformed smooth wall.
This hydraulic dent profiling process does, however, have considerable disadvantages. As this process is limited to tube and cylinder walls, it is not possible to produce large dent-profiled sheet metal or foil with variable, geometric dimensions in the dented structure. Another disadvantage of this dent profiling process is that in order to bend sheet metal into a cylindrical form with perfect roundness, the two-cylinder rounding machining, which is the best known method, is used. (In this method, sheet metal is rolled over a rigid roller, which presses on a flexible bottom roller. This method is described in, for example, German patent nos. 1602489, 1752001 and 1552017.) This well-known and old twin-cylinder rounding method is more suitable for the production of cylindrical building components requiring no structured walls. When structured and cylindrical components are to be manufactured by the twin-cylinder rounding method, the structured molds are expensive and the surface quality of the raw materials is severely degraded due to the strong mechanical deformation process.